The following description includes information that may be useful in understanding the present subject matter. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed subject matter, or that any publication specifically or implicitly referenced is prior art.
Magnetic Resonance Imaging (MRI) employs a strong magnetic field that is used to polarize the spin magnetization in a patient's body. The spin magnetization that is most often used in MRI arises from the nuclei of hydrogen atoms within the body. Although the highest concentration of hydrogen atoms within the body is found in water molecules, other compounds found in the body (e.g. lipids, glucose, etc.) are present in sufficient concentration to provide a detectable MR spin magnetization.
MR imaging of internal body tissues may be used for numerous medical procedures, including diagnosis and surgery. In general terms, MR imaging starts by placing a subject in a relatively uniform, static magnetic field. The static magnetic field causes hydrogen nuclei spins to align and precess about the general direction of the magnetic field. Radio Frequency (RF) magnetic field pulses are then superimposed on the static magnetic field to cause some of the aligned spins to alternate between a temporary high-energy nonaligned state and the aligned state, thereby inducing an RF response signal, called the MR echo or MR response signal. It is known that different tissues in the subject produce different MR response signals, and this property can be used to create contrast in an MR image. An RF receiver detects the duration, strength, and source location of the MR response signals, and such data are then processed to generate tomographic or three-dimensional images.
Loop coils are widely used for the excitation and detection of signals in MRI. Loop coils are typically designed as symmetric structures with respect to earth ground and may be connected to a coaxial cable for signal transmission. The coaxial cable itself is an unsymmetrical component, and by connecting a symmetric antenna to an asymmetric transmission line a surface current can be generated on the shield of the coaxial cable. To block this surface current, baluns are typically placed between the antenna and the coaxial cable. However, baluns are resonant structures that can interact with the loop coil.
In addition, loop coils are widely used for construction of phased arrays in MR imaging. The size of individual coils in modern arrays is relatively large, which in turn, limits the maximum achievable channel count and acceleration. To reach a very large number of elements and realize the advantages of highly accelerated imaging, the size of each loop coil should be reduced or minimized. Likewise, for accelerated imaging of small anatomy (e.g., the fingers), the size of the coil elements in an array should be small enough so that each element has a unique sensitivity profile. Regardless of the motivation for reducing coil element size, with a small loop size, the placement of the feed circuitry, including the balun, becomes a limiting factor in the design. A conventional approach may be problematic because the components and traces may interact and cover adjacent elements.